WRIGHT AND HOSTETTER: CRYSTAL GROWTH 411 



the atoms and group of atoms on deposition enter into the crystal 

 state in practically the same condition of strain as that of the 

 orienting crystal particles to which they become affixed. In view 

 of the increased size of the crystal there is of course a general 

 decrease in strain throughout the crystal but this affects the en- 

 tire crystal (both new and old parts) and is so slight that it is 

 not appreciable in the phenomena presented by the crystal when 

 examined in polarized light. The newly deposited layers are not 

 deposited in a state corresponding to that of the unloaded crys- 

 tal (isotropic state) and there is no line of demarcation in strain 

 between the original crystal and the freshly deposited layers. 



Experiments. The method of observation employed in these 

 experiments is essentially that first used by Brewster 11 in measure- 

 ments of the relative strain in glass. Brewster discovered that 

 a plate of glass under load is birefracting and that the optical 

 effect is sensibly proportional to the intensity of the strain 

 (load). The problem was studied later by Fresnel, 12 F. E. Neu- 

 mann, 13 Wertheim, 14 Mace de Lepinay, 15 Kerr, 16 Pockels, 17 and 

 recently by F. Coker, 18 and F. E. Wright. 19 As a result of 

 Brewster's law that the birefringence, or path difference, is pro- 

 portional to the strain, the determination reduces to the simple 

 determination of the path difference between the two waves 

 transmitted through the stressed crystal in a direction normal 

 to the direction of the applied load, the faster wave vibrating 

 parallel to the direction in which the load is applied, the slower 

 normal to this direction. If white light be used as source of light 

 the path difference gives rise to interference colors which follow 

 approximately the Newton color scale. For qualitative obser- 

 vations uniformity of interference tint over the field near the 

 edge under observation is a criterion for uniformity of strain in 



1 Philosophical Transactions 1814, 1815, 1816. 



2 Oeuvres Completes. 



3 Pogg. Ann., 54. 1841. 



4 Comptes Rendus, 32, 33, 34. 1854. 



5 Ann. chim. phys., 19: 1-90. 1880. 



6 Phil. Mag., (5) 36: 321. ' 1886. 



7 Ann. d. phys., 7: 745-771. 1902; 9: 220-223. 1902. 

 3 Phil. Mag., 20: 749. 1910. 

 9 J. Wash. Acad. Sci., 4: 595-598. 1914. 



